1. Field of the Disclosure
Embodiments of the present disclosure relates to a large scale energy storage system, more particularly, to a large scale energy storage system which may enable balanced control operations between inverters so as to balance inverters modules connected in parallel.
2. Background of the Disclosure
With recent global industry acceleration, energy demands are consistently increasing and continuous usage of fossil fuel is increasing carbon dioxide emissions, only to bring serious weather change. Engineering solutions for such problems are needed.
A distributed power supply and spread policy is positively pushed in accordance with recent government lead green growth vision. Sudden rise of oil price leads investments of big businesses and small and venture businesses so that relevant markets are rapidly expanded.
However, new regeneration energy can be seriously affected by location environment and natural conditions and output variation of the new regeneration energy is fluctuated, so that it may be impossible to supply the new regeneration energy continuously and that a time difference between energy production and energy demand may be generated. From a long-term point of view, there is a limit in the current power system's absorption of new regeneration energy variation. An alternative for controlling the variation of the new regeneration energy is needed.
An electric energy storage system comes to the force as a good method for dealing with intermittent supply of the new regeneration energy in a short time period.
Especially, the energy storage system charges electricity when the power amount is large and discharges electricity when the power consumption is large, only to reduce a difference between power supply and power demand efficiently. Energy storage technology stores the generated electricity uses the electricity when needed, and the energy storage technology is recognized as key technology which will lead the future energy market as a device for enhancing energy use efficiency, for raising new regeneration energy usage and for stabilizing the electric energy storage system.
When using a large scale energy storage apparatus, load factor improvement, peak shaving and electric load leveling are promoted by solving the daily electric load difference. Also, output stability of distributed power and new regeneration energy and improvement of electricity quality can be supported and emergency power supply and high quality power supply can be supported.
FIG. 1 is a conceptual diagram schematically illustrating a range of application of a conventional energy storage system.
The conventional energy storage system shown in FIG. 1 stores the energy generated in an electric power system and uses the stored energy at a time of necessity. An energy storage system installable in a transmission and distribution network is a MW class or larger scale energy storage apparatus. Such the energy storage system may be applicable to diverse fields such as new regeneration energy, industrial facilities, commercial facilities, residential areas and electric power systems.
FIG. 2 is a schematic diagram of a large scale energy storage system in accordance with the prior art and FIG. 3 is a waveform diagram of an inverter module shown in FIG. 2.
The large scale energy storage system shown in FIG. 2 includes a plurality of inverter modules 10 connected in parallel and the plurality of the inverter modules 10 are configured to convert a DC voltage between a DC of a battery (BATT) and a grid into a AC voltage to output the AC voltage.
Between an output terminal of each inverter module 10 and the grid may be arranged an reactor 11 configured to restrict the output currents of the inverter module 10 so as to suppress harmonics from outputting to a power supply and a filter 12 configured to filter the noise generated from the power supply or the noise generated from the inverter.
When switching timing signals (PWM11, PWM12, PWM21 and PWM22) between switch devices (Q11, Q12, Q21 and Q22) provided in the inverter module 10 are different from each other, especially, in case an independent control circuit is provided in each inverter module 10, in the operation of the large scale energy storage system having the structure mentioned above as shown in FIG. 3, “+”-sided switch device Q11 of any one inverter module 10 is switched on and “−”-sided switch device Q22 of another inverter module 10 is switched on. In this instance, a high frequency circulating current is generated by switching between the two inverter modules 10. Current imbalance between the inverter modules 10 is caused by property variation of the switching element, reactor and filter or property variation caused by temperature variation, such that a low frequency circulating current might flow between the inverter modules.
The inverter modules 10 have to be connected in parallel between the DC of the battery and the grid. Accordingly, a driving method of each inverter module 10 or property variation of the inverter modules 10 might cause mutual interference and current imbalance.
In addition, the capacity of the large scale storage system might deteriorate and there might be damage to the switching elements of the inverter modules. Property variation of the inverter modules may be generated by property variation of the switching elements and switching timing, property variation of reactors and filters, and property variation of devices according to temperature variation.
The present disclosure is derived to solve the disadvantages mentioned above and it provides a large scale energy storage system which may operate balanced control of inverter modules connected in parallel so as to control balancing of the inverter modules without signal distortion caused by affection of noise.